1. Field of the Invention
The present invention relates to a method and a device for detecting deterioration of automotive vehicle oil, such as engine oil, using a pH sensor which generates an output in response to the degree of the acidity or basicity of the oil, and a method and a device for warning a vehicle user of detected oil deterioration.
2. Description of the Related Art
A conventional pH sensor has been known for detecting the acidity or basicity of an aqueous solution directly and continuously using electrodes. This conventional pH sensor includes a reference electrode presenting a constant electrode potential, and a pH electrode presenting an electrode potential changing in response to the acidity or basicity of the aqueous solution. The pair of electrodes, when immersed in an aqueous solution to be measured, develop a potential difference, substantially proportional to the degree of the acidity or basicity, between them. By measuring this potential difference with a potentiometer built in the sensor, the acidity or basicity of the aqueous solution is detected. Therefore, the dispersion of he electrodes causes different outputs to be generated for the same acidity or basicity of the aqueous solution. In other words, different pH sensors have different output characteristics.
Japanese Unexamined Patent Publication No. 56-47614 discloses an automatic oil changer using the aforementioned pH sensor for detecting the basicity of the engine oil. Further, Japanese Unexamined Patent Publication No. 3-175350 discloses an electrode for improving the reliability of the pH sensor in the oil.
In the device disclosed in the former one of the aforementioned patent publications, the oil is automatically changed when the basicity of the oil decreases to less than a predetermined threshold value. This threshold value is not set by obtaining an output characteristic of the pH sensor with respect to the mileage, i.e. the distance that has been covered by the vehicle. Therefore, the timing of oil change can be affected by output characteristics of the pH sensors. The latter patent publication also deals with electrode materials and a combination thereof but not the output characteristic of the pH sensor with respect to the mileage.
Specifically, in order to prevent engine performance from being deteriorated by sludge (insoluble substance) resulting from oxidization of engine oil, the engine oil contains additives such as an antioxidant and a detergent-dispersant. With the progress of consumption of these additives, the oil comes nearer to the end point of its service life. However, there has been no conventional method or apparatus for detecting the oil deterioration which is capable of detecting the end point of the service life of the oil or the progress of the oil condition in the vicinity of the end point of the service life more accurately, without any influence of output characteristics of the sensors, by monitoring the output characteristic of the pH sensor with respect to the mileage covered by the vehicle to perceive the behavior of the additives in the oil.
Further, Japanese Unexamined Patent Publication No. 6-201649 discloses a device in which the degree of deterioration is judged based on a total acid value map which has been determined for every combination of the electrode materials to be used. However, this device does not judge the degree of deterioration by monitoring a change rate of the output from the pH sensor for the driving mileage or driving time. In addition, Japanese Unexamined Patent Publication No. 4-350552 discloses a device which judges that the oil has deteriorated when the sensor output has changed from a decrease into an increase. However, the device does not judge the deterioration of the oil by detecting a change in the sensor output from a condition of a large change rate into a condition of a small change rate.